The present invention relates generally to three-dimensional (3D) computerized tomography (CT). More particularly, the present invention relates to a multi-resolution area detector for such imaging and the use of such a detector in a system.
In conventional computerized tomography for both medical and industrial applications, an x-ray fan beam and a linear array detector are used. Two-dimensional (2D) imaging is achieved. While the data set may be complete and image quality is correspondingly high, only a single slice of an object is imaged at a time. When a 3D image is required, a stack of slices approach is employed. Acquiring a 3D data set one 2D slice at a time is inherently slow. Moreover, in medical applications, motion artifacts occur because adjacent slices are not imaged simultaneously. Also, dose utilization is less than optimal because the distance between slices is typically less than the x-ray collimator aperture, resulting in double exposure to many parts of the body. In 2D CT, the scanning path of the source is often a simply circular scan about the object. The linear array detector is fixed relative to the source. (Although it is usual to talk about a scan path of a source relative to the object to be imaged, it is to be appreciated that the object may be rotated or otherwise moved to provide relative motion between the object and the source.)
In a system employing true cone beam geometry for 3D imaging, a cone beam x-ray source and a 2D area detector are used. An object is scanned, preferably over a 360.degree. angular range, either by moving the x-ray source in a scanning circle about the object or by rotating the object while the source remains stationary. In either case, the area detector is fixed relative to the source. The relative movement between the source and object which is to be imaged provides scanning in either case. Compared to the conventional 2D stack of slices approach to achieve 3D imaging, the cone beam geometry has the potential to achieve rapid 3D imaging of both medical and industrial objects with improved dose utilization.
When imaging a relatively large object using 3D CT, one must use a relatively large area detector. Since relatively large area detectors tend to have rather poor spacial resolution, the quality of image produced using such an arrangement may not be satisfactory for some purposes. The difficulty in producing relatively large area detectors having high spacial resolution at reasonable cost is at least partly a result of complexities and difficulties in providing a relatively large number of detector elements within an area detector.
If a relatively large area detector will not provide satisfactory resolution when imaging a relatively large object, a relatively small, high spacial resolution, high quality area detector may be used in order to image a region of interest. However, use of such a relatively small area detector in order to image a region of interest (which is less than the complete object) provides only incomplete data (with data corruption) such that significant artifacts are present in the images. Moreover, such techniques usually image only the region of interest. Although the region of interest is the most important part of the object being viewed, it may also be helpful to have at least some imaging of the remainder of the object.
U.S. patent application Ser. No. 07/998,330, filed Dec. 30, 1992, in the name of Jeffrey W. Eberhard, Kwok C. Tam, and Kristina H. Hedengren, entitled "THREE DIMENSIONAL COMPUTERIZED TOMOGRAPHY SCANNING CONFIGURATION FOR IMAGING LARGE OBJECTS WITH SMALLER AREA DETECTORS," now U.S. Pat. No. 5,319,693, assigned to the assignee of the present application, and hereby incorporated by reference, discloses a technique for using a relatively small, high-resolution, high-quality, area detector in order to simulate a relatively large, high-resolution, high-quality area detector. The technique allows one to image parts which are too large for the area detector.
Even if one is able to obtain high resolution CT data for a relatively large object from an area detector, such high resolution data for a relatively large object will require great demands for data processing. In other words, computer processing power and computer memory requirements will be relatively high.
Inspection of large objects with high resolution requires massive data sets and lengthy reconstruction times which limit the usefulness of such inspections. Thus, even if one is able to overcome the difficulties of imaging large objects with high resolution, data processing requirements and related factors may cause further difficulties.